I see Mozilla have released RC2 of Firefox 1.5. RC2 means release candidate 2 – in other words this is not a completed product, it is released as a beta. The release notes for Firefox 1.5 note that RC2 gives several fixes to automated update system over RC1. While RC1 has lots of updates over Firefox 1.0 as you’d expect, as noted here previously.
If you want to try it out, the download links are on the Mozilla Firefox Project page.